The bleaching of glyceride esters of fat-forming acids (collectively "oils" herein) often is done with a clay such as an acid clay. The clay becomes spent, i.e., substantially incapable of further economic sorption of color bodies or of other economic use. (Such clay also is used to bleach fatty acids or as a catalyst to assist in polymerizing unsaturated fatty acids to make "dimer " acids and higher polymers.) Frequently such spent clay is mixed with some activated carbon, and the presence of such activated carbon can be tolerated by the instant process.
Usually the spent clay is fltered from oils in the process of their refining, yielding a cake that often has about 20-55 % oil content (mainly glyceride esters) and the balance essentially clay solids. Disposal of such cake as landfill often conflicts with environmental considerations because of oily drainage and/or undesirable degradation or inflation of the oil present. Such perfunctory disposal can cost more than three dollars a cubic yard for hauling, and it also represents a waste of recoverable glyceride esters and/or fatty acids of fair value.
The desirability of recovering as esters various saponifiable oils (which are mostly glyceride esters) from spent bleaching clay has been evident for many years. The U.S. patent art reflects a variety of organic solvent extraction treatments and a variety of aqueous treatments for this purpose. The latter proposals include the use of various surface tension-reducing agents in the water to assist the oil separation or "washing" treatment of the spent clay, for example, added synthetic detergents, sodium aluminate, sodium stannate, sodium fluoride, water-soluble coagulant salts and added or in situ-produced sodium soaps of fatty acids. The less pertinent art on the aqueous washing of spent clays is directed to substantially complete saponification of the oils with caustic soda or soda ash at temperatures approaching the atmospheric boiling point of water. The two heretofore patented proposals described below are believed to be the ones to be the most pertinent to the instant process.
U.S. Pat. No. 1,828,035 shows spent clay washing with aqueous saline solution containing caustic soda or caustic potash sufficient for neutralizing the free fatty acids present, but insufficient for saponifying any appreciable proportion of the glycerides present. Such saponification, it is stated, then would cause the emulsification of glycerides and create difficulty in separating same from the aqueous phase. Free fat-forming ("fatty") acids are completely neutralized at pH about 8. Sodium chloride is stated to be replaceable by sodium sulfate or other salts tending to repress emulsification of fatty oils in the aqueous liquid.
U.S. Pat. No. 2,706,201, to the extent it is directed to separating saponifiable oils from spent clay, shows adding sodium carbonate to a boiling mixture of clay and water "until a distinct alkalinity to phenolphthalein persists." Such pH indication starts at 8.3, although this would take a highly trained eye to notice it, and the red color is reasonably evident to most users by pH 9. In augmentation of this treatment the patent also suggests adding a synthetic detergent or soap. Many such saponifiable oils contain a very small proportion of free fatty acids; their neutralization to form sodium soaps under these conditions is unavoidable. The patent goes on to point out that where the required approximately 1% of soap cannot be formed in this way, it can be obtained by deliberately saponifying some of the ester with a bit of caustic soda.
While it is not absolutely clear from these references just how much of the glyceride esters present on the spent clay are recovered as glycerides, nor just how clean the clay becomes from such treatment, it should be fairly evident that some of the glyceride esters present are recoverable by such processing.
The instant invention is based on the discovery that for domestic glycerides a "pH window" exists as a rather sharply defined salient in the aqueous caustic washing of the spent clay, and additionally surprising, such salient is at a pH very much higher than previously taught or suggested by the art.